This invention relates to resistance welding apparatus which affords protection to an operator who inadvertently exposes any part of his hand to the risk of crushing between the welding electrodes of the apparatus.
It is a fundamental requirement of resistance welding apparatus that the welding electrodes be applied to a workpiece under load. Generally, the load required for a satisfactory weld is in the range of a few hundred pounds to several tons per square inch; such high loads will instantly crush a finger or thumb.
In an earlier construction of resistance welding apparatus affording operator protection, a power actuated welding electrode makes a low-force approach to within a quarter of an inch from a workpiece before it is switched to a high-force operating mode. If the electrode meets an obstruction such as a finger during its low-force approach, then it is quite simply halted by that obstruction and no serious injury results. In the earlier apparatus, the moment at which the power actuated electrode is switched to its high-force operating mode is determined by the position of a switch that is actuated by a piston driving the electrode towards the workpiece. Since the welding electrodes tend to become shortened by wear and re-dressing in service, a condition can arise in which the power actuated electrode is more than a finger thickness from the workpiece at the moment it is switched to its high-force operating mode. To make the apparatus safe again the operator must either adjust the position of the mode switch, or replace the electrodes with new ones; this gives rise to problems of safety maintenance.